From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mga02.intel.com (mga02.intel.com [134.134.136.20]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ml01.01.org (Postfix) with ESMTPS id A32182194D387 for ; Mon, 3 Dec 2018 12:53:42 -0800 (PST) Message-ID: <2a3f70b011b56de2289e2f304b3d2d617c5658fb.camel@linux.intel.com> Subject: Re: [PATCH RFC 2/3] mm: Add support for exposing if dev_pagemap supports refcount pinning From: Alexander Duyck Date: Mon, 03 Dec 2018 12:53:42 -0800 In-Reply-To: References: <154386493754.27193.1300965403157243427.stgit@ahduyck-desk1.amr.corp.intel.com> <154386513120.27193.7977541941078967487.stgit@ahduyck-desk1.amr.corp.intel.com> <97943d2ed62e6887f4ba51b985ef4fb5478bc586.camel@linux.intel.com> Mime-Version: 1.0 List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: linux-nvdimm-bounces@lists.01.org Sender: "Linux-nvdimm" To: Dan Williams Cc: =?ISO-8859-1?Q?J=E9r=F4me?= Glisse , Barret Rhoden , "Zhang, Yu C" , KVM list , linux-nvdimm , Jan Kara , David Hildenbrand , Linux Kernel Mailing List , Linux MM , rkrcmar@redhat.com, Paolo Bonzini , Christoph Hellwig List-ID: On Mon, 2018-12-03 at 12:31 -0800, Dan Williams wrote: > On Mon, Dec 3, 2018 at 12:21 PM Alexander Duyck > wrote: > > > > On Mon, 2018-12-03 at 11:47 -0800, Dan Williams wrote: > > > On Mon, Dec 3, 2018 at 11:25 AM Alexander Duyck > > > wrote: > > > > > > > > Add a means of exposing if a pagemap supports refcount pinning. I am doing > > > > this to expose if a given pagemap has backing struct pages that will allow > > > > for the reference count of the page to be incremented to lock the page > > > > into place. > > > > > > > > The KVM code already has several spots where it was trying to use a > > > > pfn_valid check combined with a PageReserved check to determien if it could > > > > take a reference on the page. I am adding this check so in the case of the > > > > page having the reserved flag checked we can check the pagemap for the page > > > > to determine if we might fall into the special DAX case. > > > > > > > > Signed-off-by: Alexander Duyck > > > > --- > > > > drivers/nvdimm/pfn_devs.c | 2 ++ > > > > include/linux/memremap.h | 5 ++++- > > > > include/linux/mm.h | 11 +++++++++++ > > > > 3 files changed, 17 insertions(+), 1 deletion(-) > > > > > > > > diff --git a/drivers/nvdimm/pfn_devs.c b/drivers/nvdimm/pfn_devs.c > > > > index 6f22272e8d80..7a4a85bcf7f4 100644 > > > > --- a/drivers/nvdimm/pfn_devs.c > > > > +++ b/drivers/nvdimm/pfn_devs.c > > > > @@ -640,6 +640,8 @@ static int __nvdimm_setup_pfn(struct nd_pfn *nd_pfn, struct dev_pagemap *pgmap) > > > > } else > > > > return -ENXIO; > > > > > > > > + pgmap->support_refcount_pinning = true; > > > > + > > > > > > There should be no dev_pagemap instance instance where this isn't > > > true, so I'm missing why this is needed? > > > > I thought in the case of HMM there were instances where you couldn't > > pin the page, isn't there? Specifically I am thinking of the definition > > of MEMORY_DEVICE_PUBLIC: > > Device memory that is cache coherent from device and CPU point of > > view. This is use on platform that have an advance system bus (like > > CAPI or CCIX). A driver can hotplug the device memory using > > ZONE_DEVICE and with that memory type. Any page of a process can be > > migrated to such memory. However no one should be allow to pin such > > memory so that it can always be evicted. > > > > It sounds like MEMORY_DEVICE_PUBLIC and MMIO would want to fall into > > the same category here in order to allow a hot-plug event to remove the > > device and take the memory with it, or is my understanding on this not > > correct? > > I don't understand how HMM expects to enforce no pinning, but in any > event it should always be the expectation an elevated reference count > on a page prevents that page from disappearing. Anything else is > broken. I don't think that is true for device MMIO though. In the case of MMIO you have the memory region backed by a device, if that device is hot-plugged or fails in some way then that backing would go away and the reads would return and all 1's response. Holding a reference to the page doesn't guarantee that the backing device cannot go away. I believe that is the origin of the original use of the PageReserved check in KVM in terms of if it will try to use the get_page/put_page functions. I believe this is also why MEMORY_DEVICE_PUBLIC specifically calls out that you should not allow pinning such memory. - Alex _______________________________________________ Linux-nvdimm mailing list Linux-nvdimm@lists.01.org https://lists.01.org/mailman/listinfo/linux-nvdimm